Removal of arterial plaque and blood clots by ultrasound is a new atherectomy method, taking its place as an important blood vessel cleaning device. Other devices now used for this purpose work by a process of cutting, scraping or slicing the obstruction within the artery or blood vessel. All atherectomy devices are two to five feet long, and less than 3 mm in diameter, small enough to be threaded through the arterial system to the coronary arteries, or small arteries of the leg.
Unlike cutting or scraping devices which have a rotating wire transmitting motion to a spinning tip, an ultrasonic system uses a thin wire to transmit sound waves to the distal working end. The wire, when tuned to the generator frequency, maintains a standing sound wave down its length. This wave is a compressional, axial, motion, where the molecules are stressed by compression and tension at the nodes or point of no movement, and the molecules are moving back and forth at great speed through the points known as anti-nodes.
When the distal tip of the wire vibrates in liquid such as blood, if the amplitude of movement is large enough, it causes cavitation, the making and breaking of bubbles. These bubbles are microscopic and extremely numerous. They collapse with great force producing a shearing and tearing action on any material in the bubble field. The large forces involved, although on a micron level, can, for example, in a mechanical system, cause boat propellers and pump cylinders to erode.
In order to negotiate the sinuous turns of the blood vessels, especially the curves entering the coronary arteries, the wire must be made thin. This small size provides the necessary flexibility so that the wire can be threaded easily to the occlusion. As the wire diameter is reduced, however, below approximately 0.030" (3/4 mm), a problem arises. There appears a degree of freedom for lateral or transverse motion in the wire which is detrimental to the operation of the system. This tendency toward perpendicular vibration reduces the axial, or forward and backward, motion of the wire tip and also produces fatigue in the wire. This fatigue weakens the wire and tends to break it. It can be easily understood that if a thin wire touches an obstruction, such as arterial plaque, the wire would be especially vulnerable to whipping motion since instead of going forward into the occlusion, the wire more easily moves sideways. The reduction of axial motion results in lowered cavitation at the wire tip and reduces plaque or clot removal. For reasons of both danger to the wire integrity, as well as the reduction of cavitation intensity, it is vital to suppress the unwanted transverse motion.
In prior pending U.S. patent application Ser. No. 07/632,679, transverse motion is suppressed by a close fitting catheter or sheath surrounding the wire, and also a liquid flowing between wire and sheath. However, as the wire must be made even thinner and more flexible, the unwanted vibration problem again arises since the wire more easily tends to shiver.